Update packages/tower-middleware/server/agent.coffee

fix exception:
TypeError: Object Mac OS X 10.8.2 has no method 'match'
    at Object.Tower.MiddlewareAgent [as handle] (/home/erundook/towertest/node_modules/tower/packages/tower-middleware/server/agent.coffee:14:23)
Merge pull request #376 from erundook/patch-1

fix agent detection bug: TypeError: Object Mac OS X 10.8.2 has no method 'match'